Because of numerous desirable features such as lightness, high strength to weight ratio, and relatively low cost, plywood panels have long been used for structural purposes such as forms in casting concrete structures and as siding or paneling. However, plywood panels for such purposes have had to be prepared with a high grade veneer surface which is defect free. Panels used in poured concrete forming require smooth surfaces, since any surface irregularities will appear on the surface of the concrete. Moreover, raw wood panels used in the forming of concrete absorb moisture and as a result undergo surface deterioration and even buckling or warpage so that their useful life is lessened. Panels intended for use as siding requires a surface which is weather resistant. This surface is traditionally achieved by painting. The raw wood surface of panels is highly absorbent, however, so that a large quantity of paint must be applied before a suitable surface finish is obtained and such surface is still subject to checking.
In the past, attempts have been made to overcome these disadvantages. Such attempts include providing the panel with a resin impregnated surface layer of kraft paper or otherwise providing a plastic resin surface on panels. Despite these attempts, none of the prior processes has been wholly successful in providing a protective surface on plywood panels which have surface defects. Each of the prior processes and resulting panels suffer from one or more disadvantages.
A variety of the prior panel surfacing methods employ thermosetting resins to fill cracks and fissures in the panel surface. Thermosetting resins are difficult to use because they set up with heat and are no longer workable as a liquid thereafter. Moreover, a thermosetting resin layer shrinks and changes shape as it sets. Thus, a layer of thermosetting resin thicker than is actually desired in the final product must be applied to allow for the shrinkage. Also, it is necessary to use mechanical finishing procedures, such as sanding, to obtain a smooth surface after a thermosetting resin is solidified by the application of heat. Any excess thermosetting resin removed from a panel surface during such a finishing procedure is an unusable waste product.